1. Field of the Invention
This invention generally relates to systems configured to generate output corresponding to defects on a specimen. Certain embodiments relate to systems that include an optical subsystem that is configured to create interference between a test beam and a reference beam, both of which are reflected from a specimen.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a semiconductor wafer and then separated into individual semiconductor devices.
Inspection processes are used at various times during a semiconductor manufacturing process to detect defects on a specimen such as a reticle and a wafer. Inspection processes have always been an important part of fabricating semiconductor devices such as integrated circuits. However, as the dimensions of semiconductor devices decrease, inspection processes become even more important to the successful manufacture of acceptable semiconductor devices. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary since even relatively small defects may cause unwanted aberrations in the semiconductor devices.
Many different types of inspection tools have been developed for the inspection of semiconductor wafers. Defect inspection is currently being performed using techniques such as bright field (BF) imaging, dark field (DF) imaging, and scattering. Phase detection is typically performed using spatial fringe modulation. The type of inspection tool that is used for inspecting semiconductor wafers may be selected based on, for example, characteristics of the defects of interest and characteristics of the wafers that will be inspected.
There are, however, many disadvantages to currently used inspection systems. For instance, as design rules shrink, the amplitude perturbations and complex fields resulting from defects are significantly weaker compared to those resulting from the object being inspected. With BF mode, because of the small amplitude perturbations, the contrast of the defect image is relatively low making the defect extremely difficult to detect. For DF mode, the defect contrast is generally satisfactory; however, the raw signal is typically so weak that the signal is not above the sensor noise. The raw signal may be increased by increasing the intensity of the illumination used for the DF mode. However, to increase the DF signal to useful levels, the required increase in the illumination level is impractical due to source availability or wafer damage risk.
Currently, defect inspection based on phase detection using the spatial fringe technique is susceptible to system noise, has higher costs for image processing, and is limited by the sampling of the fringe. For example, systems and methods that can be used for defect inspection based on phase information are illustrated in International Publication Nos. WO 2004/025379 by Thomas et al, WO 2004/025567 by Dal et al., and WO 2004/025568 by Voelki, which are incorporated by reference as if fully set forth herein. As described in these publications, a reference image is compared to an image of a target to detect defects on the target. The reference image can be an image reflected from a reference beam mirror or an image generated from a different position on the target than the target image. Therefore, these systems and methods will be particularly susceptible to noise such as that caused by system vibration and variations in focus at the different positions on the target. In addition, as described in these publications, relatively complex image processing techniques are used to reduce the non-defective aberrations between the images being compared. The image processing techniques not only increase cost and reduce throughput, but more importantly may undesirably alter the image data such that defects, and particularly defects of relatively small size, are detected with less accuracy.
Accordingly, it may be advantageous to develop a system that is configured to generate output corresponding to defects on a specimen by increasing the contrast between the output corresponding to the defects and output corresponding to non-defective portions of the specimen using an interference contrast enhancement technique thereby increasing the accuracy of the system for detecting defects, and particularly relatively small defects, while reducing the susceptibility of the system to noise, eliminating the need for time consuming and expensive image processing, and providing flexibility in the system for detecting multiple types of defects.